Ok! I drew up a model of the spindle from the 5C spin indexer in SolidWorks.
The model is dimensionally accurate to about 0.005" in all respects except the spindle taper and some minor irrelevant detail.
I also excluded the threads from the spindle nose as I have no need to model those right now.
I've attached a render of the part, plus a ZIP file containing both an STL and SLDPRT of the spin indexer spindle (from the LittleMachineShop spin indexer in case anyone else is looking for dimensions!)